Here's a simple, fun, free FPS game. It also has some really nice graphics for being open sourced.

Alien Arena
Homepage
OS:Windows, Linux, Mac(?)
Open Source:Yes!
Description:From the game's website:
Do you like fast paced deathmatch? How about rich, colorful, arcadelike atmospheres? How about...retro Sci Fi? Then you're going to love what Alien Arena 2007 has in store for you! This game combines some of the very best aspects of such games as Quake III and Unreal Tournament and wraps them up with a retro alien theme, while adding tons of original ideas to make the game quite unique.
Single player just involves you and bots in various deathmatches. But the game also supports online multiplay which adds deathball, CTF, and other team matches in addition to the deathmatch free-for-alls. Good, mindless fun.

If anyone has the interest/ability to run a server, this could be a fun one for orgahs.